1. Field of Invention
The present invention relates to a flat panel display and particularly to a thin film transistor (TFT) of a flat panel display.
2. Description of Related Art
Copper metal has advantages such as low resistance, low coefficient of thermal expansion, high melting point and good electro-migration resistance capability. Therefore, copper metal may have high potential to be used for electrode structures in a TFT.
However when used on the TFT, copper metal easily reacts with silicon to generate copper silicide, and copper metal has a high diffusion coefficient in the dielectric layer, thus easily resulting in electrical degradation of the TFT. Moreover, the poor adhesion between the copper metal and the dielectric layer easily results in peeling. In order to solve the aforementioned problem, a diffusion barrier layer is additional disposed in the copper electrode TFT structure in addition to copper metal.
In the conventional TFT structure, when copper metal is used in the source/drain electrodes of the TFT, a barrier layer must be formed between the source/drain electrodes and the amorphous silicon layer and the phosphorus doped silicon layer. In this case, the source/drain electrodes of the TFT would not directly contact the amorphous silicon layer and the phosphorus doped silicon layer so that the aforementioned problem of copper silicide generation can be avoided. However, when an oxygen-containing compound (oxide or oxynitride) is used as a barrier layer additional problem can be raised. During the subsequent process, a protection layer is formed by a Plasma Enhanced Chemical Vapor Deposition (PECVD). In this case, the barrier layer formed by the oxygen-containing compound is reduced and damaged by the reacting gas in the PECVD process, thereby producing bubbles and resulting in peeling of the copper film of the source/drain electrodes.
Due to the aforementioned problems, there is a need to provide a TFT structure so that copper electrodes can be used in the TFT structure and would not be damaged during the manufacturing process.